Two-phase pressure drop reduction bwr assembly design

ABSTRACT

In a boiling water reactor having discrete bundles of fuel rods confined within channel enclosed fuel assemblies, an improved fuel design of bundles of fuel rods interior of the channels is disclosed. Specifically, partial length rods are utilized which extend from the bottom of the channel only part way to the top of the channel. These partial length rods are symmetrically distributed throughout the fuel bundle with the preferred disposition being in the second row of the bundle of fuel rods from the channel wall. The symmetrical distribution of the partial length rods is at spaced apart locations one from another. During shutdown of the reactor, an improved cold shutdown margin is produced at the top of the fuel assembly due to the improved moderator-to-fuel ratio and reduction in plutonium formation at the upper portion of the bundle. Shutdown control rod worth is improved due to greater moderator-to-fuel ratio and a longer thermal neutron diffusion length. During power reactor operation, the partial length fuel rods improve flow distributions above the ends of the partial length rods by channeling steam in the open interstitial area between rods above the ends of the partial length rods.

BACKGROUND OF THE INVENTION

This invention relates to fuel bundles for placement in channelcontained fuel bundles for use in boiling water reactors. Moreimportantly, an improved fuel bundle is disclosed which utilizes partiallength rods extending from the bottom of the assembly to the regions ofthe fuel assembly in which two phase steamwater flow occurs. Thesepartial length rods can be symmetrically or asymmetrically distributedin the interior of the assembly, with the preferable location relativeto the channel side walls of a bundle of rods or fuel assembly,contained within a fuel channel of a boiling water reactor, depending onwhether nuclear or thermal hydraulic performance was being optimized.

SUMMARY OF THE PRIOR ART

Modern boiling water reactors include in the core region of the reactora core bypass volume and a fuel channel volume.

The fuel channel volume includes bundles of elongated rods or claddingcontaining the reacting fuel. These bundles are placed within channelsbetween a support plate on the bottom and a tie plate at the top.

The core bypass volume is exterior of the channels. It is the region inwhich the control rods control the nuclear reaction and includesadditional water moderator for efficient reaction.

Modern fuel bundle design has been limited by the need to operate belowthermal limits and avoid thermal hydraulic instabilities and couplednuclear-thermal-hydraulic reactor core instabilities. The stabilitylimits affect the degree to which the fuel assembly can be optimized forminimum fuel cycle costs. Modern boiling water reactor fuel bundledesign also is limited by the need to be able to shut the reactor downin the cold state with any control rod stuck in the fully withdrawnposition. Since one of the purposes of this invention is to optimize thefuel bundle design for fuel cycle economics while maintaining goodmargin to thermal, stability and cold shutdown limits, a discussion ofthe thermal, stability and cold shutdown limits is required.

COLD SHUTDOWN LIMITS

Modern fuel bundles tend to be most reactive when the reactor is cold orshut down. In this operating state, the inserted control rods mustabsorb the maximum quantity of slowed neutrons in order to keep thereactor from going critical. The necessary degree of safety required tokeep such reactors from criticality is referred to as the cold shutdownmargin.

In a boiling water reactor the conversion ratio is greater in the top ofthe reactor due to the effects of steam voids on plutonium production.This causes plutonium to build up at a higher rate at the top of thereactor. Uranium 235 also is depleted less and the fuel burnup issmaller at the top of the reactor. This buildup of plutonium, reductionin burnup and smaller U-235 depletion at the top of the the fuel rodswithin the fuel assemblies decreases the cold shut down margin locallyat the top of the reactor. As the prior art fuel assemblies havinguniform rod distribution from top to bottom matures with burning in areactor, it is common for the cold reactivity to be greatest at the topof the bundle in the cold operating state. Therefore, in the cold stateit is the upper portion of the reactor fuel bundles that go criticalfirst. The control system must be designed to prevent the reactor fromgoing critical at these upper fuel rod portions.

The prior art has corrected for the cold shutdown margin with theaddition of burnable absorbers, typically gadolinium, or reducedenrichment to the fuel assembly as described in Crowther et al. U.S.Pat. No. 4,629,599. Unfortunately, gadolinium, is a burnable absorberthat leaves a residual after depleting which reduces the worth of thefuel load. Location of gadolinium or reduced fuel enrichment preferablyin the top of the fuel assembly causes local reduction of power at thetop of the reactor and increases the ratio of peak-to-average power inthe reactor, which also must be kept within limits. If burnable absorbercontent of the fuel and local reduction in fuel enrichment can bereduced and the cold shutdown margin nevertheless maintained, this willreduce fuel enrichment requirement and improve fuel costs and willreduce the peak-to-average power in the reactor.

THERMAL-HYDRAULIC INSTABILITIES

Forced circulation boiling nuclear reactors are operated during start-upand other conditions with natural circulation of the reactor coolant.However, the power that can be attained with natural circulation flow islimited by instabilities that occur when the power is increased to toogreat a level. As the reactor comes on-line and heats up, the coolingfluid within the reactor begins to boil. That is to say, the moderatingwater is present with increasing amounts of generated steam. This steam,when present in large (void) fractions, can lead to thermal hydraulicinstability and coupled nuclear thermal hydraulic instabilities.

Natural circulation boiling water reactors are limited in power outputby stability limitations. Stability limits require that the core for arequired power rating be of a minimum size and this influences therequired reactor vessel size and containment size. Limits of reactorvessel size limit the power that can be produced by a naturalcirculation boiling water reactor.

The following discussion will relate to these instabilities foridentification purposes only, and so that the improvements of thisinvention can be understood.

The presence or more accurately the possibility of the presence of theseinstabilities constitute limitations on fuel bundle design. Therefore,if a design can be generated that is less sensitive to theseinstabilities, improved worth and performance of the reactor fuel can berealized through a more optimum choice of fuel assembly designparameters. The practicality and economics of designing boiling nuclearreactors with no pumps and only natural circulation of the coolant alsois improved.

When the reactor is on-line and running, the reactor can be controlledin power output by the amount of coolant or water pumped through thereactor. Control occurs from a natural equilibrium resulting from theresident void fraction of steam relative to water within the reactor.Simply stated, for a constant flow rate of moderating water as powerincreases, steam void fraction also increases. This limits the moderatorpresent and in the absence of instabilities controls power output.

As the ratio of power to the flow rate is increased instabilities can beencountered. Specifically, either thermal hydraulic channelinstabilities or coupled nuclear thermal hydraulic instabilities can beencountered. The instabilities are most likely to occur when the reactoris at natural circulation conditions and the ratio of percent of ratedpower to percent of rated flow is greater than approximately 1.7.

Thermal hydraulic channel instability is caused by the potential forseveral flow rates to occur at a constant core pressure drop. This iscaused by the strong flow dependence of the two phase pressure drop onthe pressure drop in the high steam void region at the top of the fuelbundles. If the single phase pressure drop of the bottom of the bundleis increased or the ratio of the two phase pressure drop to the singlephase pressure drop is reduced, the threshold for channelthermal-hydraulic instability is increased.

Further, coupled nuclear-thermal-hydraulic instabilities also are known.In these instabilities, feedback from steam voids to reactor power canreinforce through the delay time in the recirculation loop and throughthe dependence of core pressure drop and core flow on reactor power tocause an instability. Larger steam void fractions and larger effects ofsteam on reactivity decrease margin to the threshold for this type ofinstability.

Coupled nuclear-thermal-hydraulic instability may create either localchannel instabilities or instabilities which occupy substantial portionsor all of the core. The instabilities usually reach a limit cycle butsince reactor cooling can be affected and the reactor operation becomesvery noisy, they are avoided in reactor operations.

Both thermal hydraulic instability and coupled nuclear-thermal-hydraulicinstability are sensitive to the ratio of single phase pressure drop totwo-phase pressure drop in the fuel bundle. This ratio can be easilyunderstood. As coolant is forced upwardly through a fuel bundle, thereis a pressure drop. A portion of this pressure drop occurs at the bottomnon-boiling portion of the reactor. This portion of the pressure drop isthe single phase pressure drop. The remaining portion of the pressuredrop occurs in the upper boiling portion of the fuel bundle. Thisportion of the pressure drop in the fuel bundle is a two-phase pressuredrop portion.

By reducing the pressure drop in the two-phase region relative to thesingle-phase region, the tendencies for thermal hydraulic instabilityand coupled nuclear thermal hydraulic instabilities can be reduced.

One of the variables in reactor design is to increase the number of rodsin the fuel assembly to increase heat transfer area. Unfortunately asthe number of rods goes up, the pressure drop--both single phase andtwo-phase--also goes up. When rod density in the fuel assembly isincreased, the power/flow ratio threshold for instability is reduced.

THERMAL POWER LIMITATIONS

Fuel bundles have a power limitation known as a thermal limit. Thethermal limit defines the maximum power that a fuel bundle can outputbefore transition boiling occurs in the fuel assembly. Transitionboiling occurs when the liquid film flowing on the surface of a fuel roddries out. This causes temperature fluctuation on the surface of thefuel rods and can cause a rapid increase in fuel rod surface temperaturefor a small increase in power production. Once any part of a particularbundle has exceeded this thermal limit, the power output of remainder ofthe rods within the reactor bundle are limited by that event.

The upper portion of the fuel assembly, where the steam void content isgreatest, is where film dryout or transition boiling is most likely tooccur. The propensity for transition boiling is a function of thedetailed flow structure and distribution in the top of the fuelassembly. It is desirable to maintain the liquid film flowing up thesurface of the fuel rods and have the steam flow at a higher velocity inthe central region between fuel rods. There is a tendency, due to forceson steam bubbles in the local fluid velocity distribution between fuelrods, for the steam to migrate to the central regions between fuel rodswhere it flows at a higher velocity than the liquid flowing on the fuelrods surface. However, if fuel assembly power is increased sufficiently,the combination of greater steam formation on the fuel rod surface andless available liquid film from high steam volume fractions causeadditional dryout patches to form on the fuel rod surface. As thesepatches are formed and recovered with liquid, temperature oscillationsoccur on the fuel element surface. The bundle power at which transitionboiling first occurs is called the critical power. Limits are applied inreactor design and reactor operation to avoid exceeding the criticalpower for all fuel assemblies in the reactor.

FLOW INDUCED VIBRATION LIMITS

In boiling water reactors fluid forces originate from the flow of thecoolant past the fuel rods and from the boiling process. Fuel assemblydesigns must withstand these forces and prevent vibration of the fuelrods which would cause them to wear and fail, which would cause leakageof nuclear fission products from the interior of the rod. Fuel rodspacers and upper and lower tie plates are introduced in reactor designto support the fuel rod and thus to protect against flow inducedvibration.

PRIOR PATENT ART

Axial distribution of the moderator to fuel ratio in a reactor nothaving fuel channels is known from prior art. Specifically, in UntermyerU.S. Pat. No. 2,998,367 issued Aug. 29, 1961 a design is disclosed forthe purpose of maintaining a reactor in a greater power generation statewhen resident steam voids coupled with flow control the reactor.

U.S. Pat. No. 2,998,367 describes a boiling water reactor which has apositive steam void coefficient of reactivity in the regions of thereactor where boiling occurs and which has a moderator-to-fuel atomratio that increases along the coolant path in the direction of theflow. In one embodiment said invention incorporates the axial variationin moderator-to-fuel atom ratio by having nuclear fuel elements ofdifferent lengths disposed parallel to one another.

Relative to prior art, the within invention addresses the different caseof a boiling water reactor with a negative steam void coefficient ofreactivity, with channels surrounding each fuel assembly and withspacers to separate fuel rods within fuel assemblies. For this differentreactor design, the preferred embodiment of the within inventionincorporates a synergistic combination of features to improve reactorperformance, reliability and economics including optimum location of thepartial length rods within the fuel assembly; optimal axial shape anduranium enrichment in the partial length rods; unique design of fuel rodspacers and upper tie plate to enhance performance of the part lengthrods; optimum length of the part length rods relative to spacerplacement so as to avoid flow induced vibrations; optimum location ofthe fission gas plenum within the part length rods to avoid neutronabsorption reactivity penalties; and optimum location of the part lengthrods relative to the control rods to improve control rod worth whilealso avoiding power peaking problems with the control rod withdrawn.

SUMMARY OF THE INVENTION

In a boiling water reactor having discrete bundles of fuel rods confinedwithin channel enclosed fuel assemblies, an improved fuel design ofbundles of fuel rods interior of the channels is disclosed.Specifically, partial length rods are utilized which extend from thebottom of the channel with the boiling region only part way to the topof the channel. These partial length rods are shortened with respect tothe remaining rods and are distributed throughout the fuel bundle withthe preferred disposition being in the interior of the bundle of fuelrods from the channel. In the preferred embodiment the partial lengthrods are located within the fuel assembly and extend from the bottom ofthe fuel bundle and terminate at a spacer which is located in the twophase flow region of the fuel assembly. Preferably the part length rodlength is at least 1/2 of the total height of the fuel bundle. Duringpower operation a steam water mixture is present in the open area abovethe part length rods. However, when the reactor is shutdown in the coldstate this open area is filled with water. Consequently, the part lengthrods have a larger effect on moderator-to-fuel volume ratio in the coldstate than in the hot state which favorably aids in nuclear design ofthe fuel. During start-up of the reactor, an improved cold shutdownmargin is produced at the top of the fuel assembly due to the increasedmoderator-to-fuel ratio at the top of the fuel assembly. Shutdowncontrol rod worth is improved due to some of the moderator above thepart length rods being near the control rods where the increasedmoderation increases the number of neutrons that are transported ordiffuse to the control rod surface. During power reactor operation, thepartial length fuel rods effectively channel steam flow to the expandedinterstitial area between rods overlying the ends of the partial lengthrods. This enables a high slip ratio of steam with respect to water andincreases the density of the moderating water about the remaining rodsin the upper region of the bundle at power operation. Rod spacers andbundle tie plates are provided with larger apertures overlying thepartial length rods for further flow distribution improvement andreduction of two-phase pressure drop. Most importantly and during fullreactor power output, the total pressure drop and the pressure drop inthe two-phase region of the bundle are both reduced without substantialcorresponding degradation of the fuel assemblies thermal limits. Theratio of single phase pressure drop to two-phase pressure drop isincreased tending to increase power and flow margin for thermalhydraulic instability or coupled nuclear-thermal-hydraulic instability.

The part length rods may be located in the second row from the outsideof the fuel assembly. In this location the steam channeling above thepart length rods tends to improve the fuel assembly power that can begenerated before the onset of transition boiling.

The part length rods preferably have a short natural uranium section atthe upper end to mitigate power peaking at the top of the part lengthrods. The part length rods preferably have a bottom fission gas plenumto minimize or eliminate fission gas plenum retainer spring and hydrogengetter device neutron absorption at the top.

OTHER OBJECTS, FEATURES AND ADVANTAGES

An object of this invention is to disclose a fuel design for placementwithin a channel contained fuel assembly that has an improved coldshutdown margin. According to this aspect of the invention, the fuelbundle within a channel has partial length rods extending into andterminating within the two phase region of the fuel bundle, extendinginto the boiling region and in the preferred embodiment typicallyoccupying at least the bottom 1/2 of the bundle. The upper region of thefuel bundle above the partial length rods includes open volumes thatwould otherwise be occupied by full length fuel rods. Hence, in theupper portion of the fuel bundle in the cold state, themoderator-to-fuel ratio is increased. Consequently, with the addition ofmoderator, the cold shutdown margin is improved.

A further advantage of the resulting improved cold shut down marginenables fuel loads to be designed with reduced amounts of burnableabsorbers such as gadolinium. By reducing gadolinium, burnableabsorbers, better fuel cycle economics can be realized. The withininvention enhances and improves U.S. Pat. No. 4,629,599 by increasingthe moderator-to-fuel volume ratio in the region of the core where thecold shutdown margin tends to be poorest and by increasing the ratio ofmoderator-to-fuel ratio in the cold state to moderator-to-fuel ratio inthe hot state, locally in the top of the reactor.

Yet another advantage of this invention is that the tendency of thereactor to produce plutonium at the top of the bundle from resonanceneutron capture in uranium 238 is reduced. Specifically, the increasedmoderator to fuel ratio reduces the resonance neutron flux, the U-238neutron absorption, and the resultant plutonium production. This furtherimproves cold shutdown margin, reduces initial enriched uraniumrequirements and makes the negative steam void coefficient of reactivitysmaller. Reduced uranium 235 initial inventory improves fuel cycleeconomics and the smaller steam void coefficient of reactivity improvespressure transient response and increases couplednuclear-thermal-hydraulic stability.

An additional result of the disclosed fuel design is that the controlrod worth relative to the upper end of the bundle is improved.Specifically, the control rods acting at or near the top of the fuelbundle see a higher density of slow neutrons. This is because theambient fast neutrons are increasingly moderated by the increased volumeof water in the region above the part length rods in the second row offuel rods adjacent to the control blade. This increases the number ofthermal neutrons that diffuse or are transported to the control bladesurface where they are absorbed.

A further object of this invention is to disclose a fuel bundle designhaving maximum stability at low flow conditions. According to thisaspect of the invention, partial length rods increase the flow area inthe two phase flow region of the fuel assembly.

An advantage of this aspect of the invention is that the spatial volumeabove the partial length rods is available for steam escape from thechannel. Accordingly, these volumes draw the relatively fast moving highvolumes of steam away from the heated rods of the fuel bundle. The steamtends to concentrate and channel at a higher velocity in the open areasabove the partial length rods during power operation of the reactor.

A further advantage of steam channeling above the partial length rods isthat the rods which are adjacent to and extend above the partial lengthrods are cooled and moderated by increased liquid fraction. Theincreased liquid fraction about the remaining rods is believed to bepresent due to the steam transport to the open areas above the partiallength rods which results in the steam flowing at a high velocityrelative to the water velocity. Accordingly, these rods immersed inmoderator have improved thermal margins and reactivity and contributeadditional power. Preferred location of the partial length rods adjacentto the highest power rods in the fuel assembly and one row in fromsurrounding channel walls increases the benefit from this effect.

Yet another advantage of this invention is that the partial length rodsare located so as to shorten those full length rods that would otherwisebe limiting the power output of the fuel bundle. Since transitionboiling limits occurs in the top of the fuel assembly, this increasesallowable maximum bundle power.

Another advantage of this invention is that while the ratio of singlephase pressure drop to two phase pressure drop is reduced, the thermalmargins of a fuel bundle are preserved.

Yet another object of this invention is to disclose the construction ofthe partial length rods. According to this aspect of the invention, thepartial length rods are provided with uranium of reduced enrichmentadjacent the tip. Such a zone at the tip of the part length rod preventspower peaking at the top of the part length rod.

A further object of this invention is to disclose the construction of afuel assembly with part length rods whereby the part length rods areterminated at a fuel rod spacer. This aspect of the invention preventsflow induced vibration problems due to the part length rods notextending into and being supported by the upper tie plate.

A further aspect of this invention is to disclose the construction oftie plates and spacers for enhancing the benefits of the partial lengthrods. According to this aspect of the invention, both the tie plate andthe spacers above the partial length rods are provided with expandedflow area apertures. The apertures are made feasible by eliminating needfor some of the spacers and the upper tie plate to support fuel rods atthe locations of the partial length rods. These apertures further reducepressure drop and aid in maintaining channeling of steam above the topsof the partial length rods. These apertures allow further reduction inthe two-phase pressure drop.

An additional advantage of these apertures is that they increase thevelocity of the steam relative to the water or the slip ratio within thefuel assemblies during power generation. Increase of the slip ratioreduces the steam void content in the top of the reactor which increasesreactivity, improves fuel cycle economics and improves axial powerdistribution.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of this invention will becomemore apparent after referring to the following specification andattached drawings in which:

FIG. 1 is a perspective of a reactor.

FIG. 2 is a perspective of a fuel bundle from the reactor incorporatingthe partial length rods of this invention;

FIG. 3 is a perspective view of a single fuel bundle with the channelremoved with the fuel bundle constructed in accordance with thisinvention illustrating a 9×9 fuel channel array with partial length rodsoccupying the second row in the lower portion of the fuel bundle anddefining overlying volumes in the upper portion of the channel enclosedfuel bundle;

FIG. 4 is a plan section of FIG. 1 taken along lines 2--2 of FIG. 1illustrating in detail the placement of the partial length rods in a 9×9array;

FIG. 5A is a plan section similar to FIG. 2 illustrating in detail theplacement of the partial length rods in an 8×8 array;

FIG. 5B is a plan section similar to FIG. 5A illustrating the partiallength rods in a water cross type fuel bundle design;

FIG. 6A is a partial enlarged section of the region 6--6 of FIG. 1illustrating the bundle in a cold state illustrating the increasedmoderator to fuel ratio;

FIG. 6B is a partial enlarged section similar to the region 6--6 of FIG.1 illustrating this portion of the fuel bundle in a hot operating stateand illustrating in phantom what is believed to be a channeling of steamin the volume overlying the partial length rod, this channeling of steamcausing a reduction in the two-phase pressure drop and enabling theremaining rods in the upper region of the bundle to realize bettercooling during power generating operation, even though heat transfersurface has been removed by shortening some of the fuel rods;

FIG. 7 is a plot of the infinite lattice reactivity against moderator tofuel ratio illustrating the improved cold shutdown margin of thisinvention;

FIG. 8 is a plot of the peak to average power ratio as a function ofbundle height, wherein the reduced rod density in the upper portion ofthe fuel bundle coupled with the improved moderator to fuel ratio aresynergistically combined to improve cold shutdown ratio;

FIG. 9 is a plot of neutron power against core coolant flow rateillustrating the intersection of the lines of core flow control withminimum forced circulation and natural circulation flow rates at regionswhere both thermal hydraulic instabilities and coupled nuclear thermalhydraulic instabilities can occur in improperly designed fuel bundles;

FIG. 10 is a detail illustrating the top and bottom of a partial lengthrod illustrating the partial removal of the gas plenum to the bottom ofthe partial length rod and use of uranium of reduced enrichment at thetip of the partial length rod to avoid peaking of adjacent full lengthrods opposite the tip of the partial length rods; and

FIG. 11 is a plot of measured effects of part length rods on pressuredrop as a function of bundle power.

Referring to FIG. 1 a partial view of a reactor is illustrated so thatthe invention may be fully understood. A shield wall 21 is shownsupporting a support skirt 20 and a reactor vessel V. The vessel hasinside a core plate 17 and an overlying top guide 12. Fuel assemblies 15are placed within the core C of the reactor on core plate 17 and areheld in vertical spaced apart relationship at top guide 12. These fuelassemblies are controlled in their output by control blades 16.

Water circulation is provided to the reactor from a recirculation outlet19 to a recirculation inlet 18 by pumps (not shown). Interior of thereactor, circulation is provided by a jet pump assembly 13 which causesoutward downflow of water and forces the water flow upwardly andinwardly through the core C. When the flow passes upwardly through thecore C, it passes through the fuel assemblies 15, up through the topguide 12, through a steam separator and steam dryer assembly and then topower extracting turbines.

This invention relates to an improved fuel assembly, one of which isshown in FIG. 2 at 15.

Referring to FIG. 2, it will be understood that the fuel assembly is notshown in its true length, and instead is broken away so as to illustratethe bottom and top portions of the bundle only.

The assembly includes an upper handle H and a lower nose piece N. Achannel 25 extends upwardly from the nose piece end substantially thefull length of the fuel assembly 15.

Individual rods L are disposed in a matrix interior of the fuelassembly. These rods extend between a lower tie plate 40 and an uppertie plate 42.

The rods are normally arrayed in rows and columns. Further, and becauseof the length of the fuel assemblies (in the order of 160 inches),spacers S are placed along the length of the fuel assembly. Typically,seven spacers S roughly evenly spaced at 20 inch intervals extend fromthe top to the bottom of the fuel assembly.

As has been extensively discussed, this invention relates to a new fueldesign for placement interior of the channels 25. Referring to FIG. 3, a9×9 fuel bundle is illustrated. Specifically, fuel rods are placed in amatrix which is 9×9. Were it not for the presence of the central waterrod 44, 81 individual rods would extend the length of the matrix of fuelrods shown in FIG. 3.

Some remarks will assist in the understanding of FIG. 3. First, thelower tie plate 40 and the upper tie plate 42 are illustrated. However,the rods are cut away in a substantial portion of their overall lengthat 50. Secondly, the invention here constitutes modifying preferably theupper 1/2 of the fuel assembly. Specifically, the modification is toshorten certain of the fuel rods. These shortened fuel rods willhereinafter be referred to as partial length rods 60. Full length rods65 will thus be seen to extend the full length of the channel. Partiallength rods 60 will thus be seen to extend at least 1/2 of the length ofthe fuel assembly. This 1/2 length starts at bottom tie plate 40 andextends upwardly to and towards upper tie plate 42. However, the partiallength rods 60 never reach the full distance. Instead, these partiallength rods terminate.

It is preferred that the partial length rods be at least within thesecond row removed from the channel walls 25. In order to illustrate thelocation of these partial length rods in the perspective of FIG. 3, thefirst row of rods has been omitted from that portion of the perspectivethat is towards the viewer. What the viewer sees is the second row ofrods. It will be understood that the partial length rods referred toherein are in all cases at least in the second row inwardly of the firstrow of rods, that inward row count starting at the channel wall.

Spacers S2 are illustrated at the upward portion of FIG. 3. Thesespacers serve to brace the rods 60 and 65 in spaced apart relation asthey extend upwardly to and through the length of the fuel assembly.However, in the case of the partial length rods, these rods extendslightly beyond spacer S1. These partial length rods 60 terminate abovespacer S1.

First, and with brief reference to either FIG. 6A or 6B, it can be seenthat spacer S2 overlying the partial length rod 60 serves two functions.First, the spacer S2 braces the full length rods 65 apart one fromanother.

Secondly, spacer S2 defines an aperture 67 of increased area compared tothe normal aperture that would be present in a spacer. As willhereinafter be explained and as can be seen in FIG. 6B, this largeraperture 67 enables flow of the coolant to occurs more easily and withlower pressure drop above the partial length rods 60.

Upper tie plate 40 can likewise be modified as is shown in FIGS. 6A and6B. This upper tie plate includes a larger aperture 69. Aperture 69reduces pressure drop above the part length rods and thereby, makes iteasier for flow to occur above the part length rods.

From the top of the partial length rod 60 downwardly to and towards thelower tie plate 40, the fuel assembly is constructed as is disclosed inthe prior art.

In the illustration of FIG. 4, the use of 12 partial length rods hasbeen illustrated. The reader will understand that the total number ofpartial length rods utilized and their locations in any specific fuelassembly 15 located within core C, may be varied to meet the necessitiesof the fuel design and the particular reactor involved.

It has been normal in the past to utilize 8 by 8 arrays of rods (such asthose that will be discussed hereafter with respect to FIG. 5). One ofthe reasons for this utilization of a lesser number of rods is todecrease the pressure drop as coolant proceeds from core plate 17 to andtowards guide 12 through the fuel channel 15.

As will hereinafter be more fully developed, the partial length rodsenable the pressure drop in the two phase portion of the fuel assembly15 to be reduced. Hence, it is possible to use a more dense array ofrods such as the 9×9 array illustrated in FIG. 4. The particular arrayillustrated in FIG. 4 because of its larger density of individual rodspresents better characteristics for production of steam while minimizingthe fuel temperature.

It will also be understood, however, that this invention is applicableto other fuel assembly designs such as the 8×8 array of rods illustratedin FIGS. 2 and 5A or the water cross design illustrated in FIG. 5B.

Referring to FIG. 5A, a water rod 44 is surrounded by partial lengthrods 60B. The partial length rods 60B extend again at least 1/2 thelength of the fuel assembly. In the discussion that follows, the readerwill understand that either a 9×9 array is illustrated in FIG. 4 or a8×8 array is illustrated in FIG. 5A can apply to this invention.

It will be understood that the concepts herein illustrated areapplicable to virtually all channel enclosed fuel bundle designs. Forexample, and as schematically set forth in schematic composite diagramof FIG. 5B, the partial length rods can be utilized in the water crosssection therein illustrated.

Referring to FIG. 5B, a water cross section of a fuel channel isillustrated. A channel 25 has its interior divided into four subsections25A, 25B, 25C, and 25D. Each subsection is defined by a subchannelassembly 30 constituting a metal member bent at right angles andabutting the inside walls of the channel 25.

These subchannel assemblies form two purposes. First, they definebetween them a cross shaped area for the containment of additionalmoderator. Secondly, they each enclose discrete volumes of rods.

Subchannel volume 25A is shown without rods. Subchannel volume 25B isillustrated with two partial length rods 60C. These rods are spaced onerow in from the respective channel walls formed by channel 25 andsubchannel assembly 30.

Subchannel volume 25C illustrates that in this particular array, itmaybe desirable to place the partial length rods 60C against the wall ofsubchannel assembly 30. This is because the water cross of this designcontains sufficient moderator immediate the partial length rod and onthe opposite side of the subassembly assembly 30.

Finally, and with reference to FIG. 5B, both 4×4 and 5×5 arrays of rodsare shown within the subchannel assembly 30. For the 5×5 array, partiallength rods 60C are shown at four spaced apart locations, all thepartial length rods being one row in from the channel sides 25 and thesubchannel assembly 30. For the 4×4 array, 2 part length rods arelocated in the central 2×2 array of fuel rods.

It will be seen that the disclosed partial length rods enhance in anunusual number of ways the performance of fuel bundle designs. Theseenhancements will be discussed in terms of the respective cold operatingstate of the reactor, the low flow, low power state of the reactor andfinally the hot and substantially full power operating characteristic ofthe reactor.

COLD START UP

Referring to FIG. 6A, that portion of the fuel bundle at section 66 isillustration at the top of the reactor adjacent the partial length rod.It can be seen that because of the absence of the rod 60, thefuel-to-moderator ratio in the upper portion of the fuel assembly isincreased. That is to say water, a moderator, is present in greateramounts and fuel is present in lesser amounts in the upper portion ofthe reactor compared to the case where the fuel rods are full length.

Referring to FIG. 7, infinite lattice reactivity is plotted against themoderator-to-fuel ratio. This figure is representative of the reactorstate at the upper portion of the fuel bundle of fuel assembly 15.

The term infinite lattice reactivity can be easily understood. Presumingthere were an infinite number of fuel assemblies 15 each of which wereinfinitely long with a lattice geometry the same as the upper part ofthe fuel assembly, the plot here shown would be an accuraterepresentation of reactivity of the upper portion of any fuel bundle.

Presuming that the reactor is critical, constructed according to theprior art and in the cold state, infinite lattice reactivity would berepresented by point 100. As the reactor moved to a hot operating stateat point 102, the moderator to fuel ratio would decrease. Specifically,the presence of steam voids and to some extent the expansion of themoderator water, would decrease the moderator to fuel ratio to a point102. Plotting the points there between gives curve 103.

Turning our attention to the partial length rods, the reader willrealize that the moderator to fuel ratio is increased. Thus, in the coldstate the infinite lattice reactivity would be represented by 104 andwould be less than for a lattice with full length rods.

Presuming insertion of the control rods, one would understand thatreactivity would fall below the critical line 106. The curve 103' willbe shifted to the right relative to curve 103 because the control rodsdisplace moderator and also cause temperature and steam voids todecrease reactivity more rapidly. It can be seen, however, that point104' falls further below the critical line. An improved cold shutdownmargin 105 exists. This cold shutdown margin can readily be compared tothe cold shutdown margin 107 of the prior art. The reader willunderstand that the cold shutdown margin constitutes a limit on theenrichment that can be provided, especially that enrichment within fuelload 15 at the top of fuel assembly 15. By the increase of the coldshutdown margin at 105, improvements to the fuel design can be made.Specifically, burnable poisons such as gadolinium can be omitted orreduced in concentration from the top portion of the reactor fuelbundles or the fuel enrichment can be increased in the top of the fuelbundle.

There is an additional complication in the cold shutdown condition inprior art reactors. Specifically, during power operation, the conversionratio or plutonium production is greater at the top of the prior artfull length rod bundles. This primarily is caused by increased resonanceneutron capture in fertile uranium 238 which produces plutonium 239. Thegreater steam voids in the top of the reactor reduce neutron slowingdown and increase resonance neutron capture relative to the reducedsteam void regions in the lower regions of the reactor. The greatersteam voids also reduce fuel burnup in the top resulting in greaterfissile material inventory and fewer fission products in the highersteam void regions at the top of the reactor core. The combination ofreduced burnup, fewer fission products, greater fissile uranium andgreater fissile plutonium gives the upper portion of the bundle a highercold reactivity. Consequently, the top of the reactor is most reactivein the cold state and the control rods must provide sufficient controlto prevent the top of the reactor from going critical.

Referring briefly back to FIG. 6A, again a higher ratio of moderator ispresent with the partial length rods of this invention. Consequently,fast neutrons have a shorter mean slowing down length. They aremoderated and fewer neutrons participate in plutonium formation throughresonance capture in uranium 238. Referring to FIG. 8, the cold neutronflux 110 of the present invention provides a lesser peak than the coldneutron flux 115 of the prior art. In other words, by suppressing theformation of plutonium, the cold neutron flux peaking normally seen atthe top of fuel assemblies is likewise suppressed.

The improved moderation of neutrons at the top of the fuel assembly hasan additional advantage. The cruciform control rods gather slow neutronsby their capture. Neutrons are slowed down in the water moderator regionabove the part length fuel rods where they have greater opportunity todiffuse to the surface of the control rod where they are captured.Because these moderated neutrons are present at greater relative levels,the control rods 16 tend to capture the moderated neutrons, againproviding superior control to the reactor in the cold state.

LOW FLOW, LOW POWER

Referring to FIG. 9, a simplified explanation of the BWR flow/poweroperating domains can be offered. Referring to FIG. 9, present ratedthermal power is plotted against percent rated core coolant flow rate.When a reactor is initially started up, it is operated with naturalcirculation of the coolant. That is the circulation that is caused bythe heating of the water interior to the reactor. As the core coolantreaches approximately 30% of rated flow rate, thermal power will be inthe range of approximately 50% of rated. This condition also can bereached if there should be a loss of power to the pumps when the reactoris operating at rated power, 116. The BWR self regulates, since, if theflow is reduced, steam voids increase which cause the reactor power todecrease along the flow control line, 117. Instabilities can occur inregions 1 and 2 of the power flow map. These instabilities includethermal hydraulic instabilities and coupled nuclear thermal hydraulicinstabilities.

As core coolant flow rate is increased, power likewise increases. Aboveapproximately 45% of the rated flow rate, on the rated flow controlline, 117, instabilities normally would not occur.

In the plot here shown, regions of instability in power versus flow arespecifically identified. Referring to region 1, it is current regulatorypractice not to allow a reactor to be operated for any significantperiod of time within this flow rate and power output region. It will benoted that this region is above the line of 40% power and falls roughlybetween 30 and 40% core coolant flow rate. Similarly, the area between40% flow rate and 45% flow rate is an area of potential instability.Reactors operated in this region must be closely monitored according tocurrent regulatory practice.

The instabilities that must be considered in fuel design and in reactoroperation were previously described and include thermal hydraulicinstability and coupled thermal nuclear hydraulic instability.

It is known that the allowable operating power at low flow as limited bystability is increased when the ratio of single phase pressure drop totwo phase pressure drop is increased.

Measurements and analysis have shown that the enclosed design reducestwo phase flow pressure drop. This is illustrated in FIG. 11 which showsthe measured change in pressure drop between a fuel assembly withpartial length rods and a fuel assembly without part length rods as afunction of bundle power and inlet mass flux of the coolant. Thepressure drop reduction caused by the partial length rods increases withincreases in fuel assembly power and the coolant flow rate. Although themagnitude of the improvement is less for the lower flow rates associatedwith natural circulation, the percentage pressure drop improvement isalmost independent of flow.

The measurements shown in FIG. 11 did not include the benefits ofspacers and tie plates above the partial length rods having greater flowapertures. Both spacers S2 and the tie plate 40 can have largerapertures above the part length rods at respective apertures 67 and 69.This further reduces two phase pressure drop which results in betterstability margins and greater full power flow capability.

It should also be noted from FIG. 6B that when the steam channels athigher velocities to the open region above the part length rods, 80, agreater density of waster remains present surrounding the upper regionsof the full length rods, 65. This water provides cooling and providesmoderation for increased reactivity. Stated in other terms, although thesections of rod 60 are missing in the upper portion of the fuel bundle,the remaining rods have improved reactivity because they are surroundedby a higher moderator fraction. This higher moderator fraction comesfrom the steam channeling to the region above the part length rods whereit flows at a greater velocity relative to the water. At steady stateconditions the steam void volume fraction in the reactor core can bedetermined from a time average of the steam flowing in the reactordivided by the time average of total coolant or steam plus water. If thevelocity of steam is increased relative to the velocity of water thesteam void fraction is reduced in the reactor core.

FULL POWER OPERATION

BWR fuel assemblies are limited in the power they can generate by thethermal limits which protect the fuel from damage. Current BWR designpractice is to limit the power output of a fuel bundle to a conditionwhere transition boiling or liquid film dryout will not occur in anyportion of a fuel bundle.

Returning to FIG. 8 and considering the hot operating powerdistribution, it can be seen that power is highest in the bottom portionof a fuel assembly and lowest at the top.

At the same time, the steam voids are low at the bottom of the fuelassembly and increase to and towards the top. In the hot operatingcondition, since the power generation is lowest at the top of thebundle, rods removed from the top of the bundle have a reduced effect onthe overall power output. At the same time, by increased moderator tofuel ratio and reducing steam void fraction via steam channeling in thetop of the reactor, the reactivity of the top of the reactor isincreased. This improves the axial power shape sufficiently toapproximately compensate for removal of fuel by making some of the fuelrods shorter. Consequently, again it can be seen that the partial lengthrods have a desirable effect on reactor performance.

The reduction in pressure drop shown in FIG. 11 makes it possible toincrease the total core flow at rated power conditions. This bothincreases reactivity of the reactor and provides better cooling of thefuel. Consequently, it is possible to operate the reactor at greaterpower output or to improve the fuel cycle economics by loading lessfissile material in the fresh fuel.

Referring to FIG. 10, the construction of a partial length rod at ornear its tip is illustrated. Referring to FIG. 10, two full length rods65 have a partial length rod 60 placed there between. Rod 60 terminatesat end plug 120.

Typically, partial length rod 60 is provided with a plenum 122 and avolume of uranium of reduced enrichment 124 followed by regularlyenriched uranium 126 extending substantially the remaining distance ofthe rod. It has been found that reduced enrichment uranium 124 reducespower peaking at the top end of the enriched region of the part lengthrod at 126. Such peaking occurs because of the absence of the neutronabsorbing uranium and increased moderation at the top end of the partiallength rod.

Fuel rods must have a fission gas plenum to accommodate fission gasesthat are produced by burnup of the fuel. The fission gas plenums forfull length rods are located above the top of the reactor core wherethey have minimal effect on the efficiency of the nuclear reaction.Location of similarly designed plenums at the top of the partial lengthrods results in parasitic neutron capture in the steel springs and othermaterials that are used in the plenum. Consequently there is incentiveto locate all or part of the partial length fuel rod fission gas plenumat the bottom of the partial length rod, thereby minimizing anyreactivity penalty caused by neutron absorption in the plenum materials.FIG. 10 illustrates an embodiment in which there is a shortened plenumat the top of the partial length rod and second fission gas plenum atthe bottom of the partial length rod.

The reader will understand that the partial length arrays illustratedare exemplary. While we prefer arrays in which partial length rods areused symmetrically within a fuel bundle, preferably in the second rowfrom the channel wall, partial length rods could be used elsewherewithin a fuel bundle. Furthermore, in some cases, the partial lengthrods can contain gadolinium or other burnable absorbers in addition toheavy element fissile and fertile materials.

The difference between the present disclosure and that described inUntermyer U.S. Pat. No. 2,998,367 are that:

1. In the reactor herein is designed to have a negative steam voidcoefficient of reactivity in the region of the reactor where boilingoccurs (as distinguished from the positive steam void coefficient shownin Untermyer);

2. The fuel elements or fuel rods disclosed herein are grouped in fuelbundles which are surrounded by fuel channels and the shorter fuel rodsare optimally located within each fuel bundle and channel so that thethermal-hydraulic and coupled nuclear-thermal-hydraulic performance ofthe reactor are improved. The fuel channels have major impact onthermal-hydraulics of the reactor. During reactor operation a liquidfilm flows up the inside of the channel. Consequently, part length rodshave a much different effect in a BWR designed with channels than theywould in a BWR, such as that of Untermyer designed without channels.Untermyer does not include fuel channels or specify the location of thepartial length rods relative to the full length rods of the fuelchannels. This location is critical to the reactor performance and isspecified herein.

3. In the reactor herein, the fuel rods, have a unique axial shape offuel U-235 or other fissile enrichment and burnable absorber such thatpower peaking problems which would occur at the tops of the partiallength fuel rods of U.S. Pat. No. 2,998,367 are avoided. Untermyer doesnot include axial distribution of enrichment or burnable absorber.

4. In the reactor herein, the fuel rod spacers within the fuel bundleare uniquely designed to enhance the thermal-hydraulic and couplednuclear-thermal hydraulic benefits of the partial length, shorter fuelrods. Untermyer does not include fuel rod spacers or design of fuel rodspacers to enhance the performance of the partial length rods.

5. In the reactor herein, the part length rods are terminated at or justabove a fuel rod spacer. This avoids flow induced vibrations which wouldoccur in the Untermyer patent.

6. In the reactor herein, the fission gas plenum for the partial lengthrods is uniquely designed so as to minimize the reactivity penalty fromlocation of the fission gas plenum at the top of a partial length fuelrod. Untermyer does not include fission gas plenums or define uniquefission gas plenum requirements for the shorter partial length rods.

7. In the reactor herein, the partial length rods are uniquely locatedin the reactor relative to the control rods such that said partiallength rods increase the reactivity worth of the control rods when thereactor is shut down. Untermyer does not include control rods or definethe location of the partial length rods relative to the control rods.

8. In the reactor herein, the part length rods are uniquely locatedrelative to the highest power rods in the fuel assembly and relative tothe fuel channel so as to enhance the critical power or margin totransition boiling performance of the fuel assembly. Untermyer does notconsider transition boiling limits or location of the part length rodsso as to enhance performance relative to said limits.

What is claimed is:
 1. In a boiling water reactor having discretebundles of fuel rods confined within channel enclosed fuel assemblies,an improvement to a fuel bundle assembly for placement in said reactorwherein said fuel bundle includes:a fuel channel having verticallyextending walls forming a continuous channel around a fuel assemblyvolume, said channel being open at the bottom end for engagement to alower tie plate and open at the upper end for engagement to an upper tieplate: a plurality of rods for placement within said channel, each saidrod containing fissile material for producing nuclear reaction when inthe presence of sufficient moderated neutron flux; a lower tie plate forsupporting said bundle of rods within said channel, said lower tie platejoining the bottom of said channel to close the bottom end of saidchannel, said lower tie plate providing defined apertures for the inflowof water in said channel between said rods for the generation of steamduring said nuclear reaction; said plurality of fuel rods extending fromsaid lower tie plate wherein a single phase region of said water in saidbundle is defined to an upward portion of said bundle wherein a twophase region of said water and steam in said bundle is defined duringnuclear steam generating reaction in said fuel bundle; an upper tieplate for supporting the upper end of said bundle of rods, said uppertie plate joining the top of said channel to close the top end of saidchannel, said upper tie plate providing defined apertures for theoutflow of water and steam in said channel from the generation of steamduring nuclear reaction; spacers intermediate said upper and lower tieplates at preselected elevations along said fuel rods for maintainingsaid rods in spaced apart locations along the length of said fuelassembly; the improvement to said fuel rods comprising: a plurality ofsaid fuel rods being partial length fuel rods extending from said lowertie plate toward said upper tie plate, said partial length rodsterminating within the two phase region of said bundle before reachingsaid upper tie plate; and, at least two of said partial length rodsbeing separate from one another so as to define in at least twolocations in said bundle spaced apart and separate vents commencing atthe top of said partial length rods and extending to said upper tieplate, each of said spaced apart vents being immediately adjoined byadjacent full length fuel rods.
 2. The invention of claim 1 and whereinsaid partial length rod is located in the interior of the fuel assemblywith at least one full length fuel rod between said partial length rodand said continuous wall formed by said channel.
 3. The invention ofclaim 1 and wherein said channel has a rectangular section;and said fuelrods are disposed in rows and columns of rods; and said partial lengthrod is located in a row or column that is not adjacent to saidcontinuous wall formed by said channel.
 4. The invention of claim 3 andwherein said partial length rod is located one row of rods removed fromsaid continuous wall formed by said channel.
 5. In a boiling waterreactor having discrete bundles of fuel rods confined within channelenclosed fuel assemblies, an improvement to a fuel bundle assembly forplacement in a reactor, said fuel assembly having water moderator pumpedtherethrough, the water moderator being liquid in the single phase lowerportion of said assembly and liquid and steam in an upper two phaseportion of said assembly, said fuel assembly including:a fuel channelhaving vertically extending walls forming a continuous closed channelaround said fuel assembly, said fuel channel being open at the bottomend for engagement to a lower tie plate and open at the upper end forengagement to an upper tie plate; a plurality of fuel rods for placementwithin said channel, each said fuel rod containing fissile material forproducing nuclear reaction when in the presence of sufficient moderatedthermal neutron flux; said nuclear reaction producing heat, said heatgenerating steam in the upper two phase portion of said fuel bundleassembly; a lower tie plate for supporting said rods within saidchannel, said lower tie plate joining the bottom of said channel at thelower open end; said lower tie plate providing defined apertures for theinflow of water moderator interior of said channel between said rods;said plurality of fuel rods extending from said lower tie plate whereina single phase region of said water in said bundle is defined to anupward portion of said bundle wherein a two phase region of said waterand steam is defined in said bundle during nuclear steam generatingreaction in said fuel bundle; an upper tie plate for supporting theupper end of said plurality of fuel rods, said upper tie plate joiningto top of said channel at the upper open end; said upper tie furtherproviding defined apertures for the outflow of water and steam moderatorinterior of said channel between said rods; spacers intermediate saidupper and lower tie plates at preselected elevations along said fuelrods for maintaining said rods in spaced apart locations along thelength of said fuel assembly; a plurality of said fuel rods beingpartial length fuel rods extending from said lower tie plate toward saidupper tie plate, said partial length rods terminating within the twophase region of said bundle before reaching said upper tie plate; atleast two of said partial length rods being separate from one another soas to define in at least two locations in said bundle spaced apart andseparate vents commencing at the top of said partial length rods andextending to said upper tie plate, each of said spaced apart vents beingimmediately adjoined by adjacent full length fuel rods; and said partiallength rod terminating at the vicinity of a spacer.
 6. The invention ofclaim 5 and wherein said partial length rods are located within saidfuel bundle with at least one rod between the partial length rods andthe wall of said channel.
 7. The invention of claim 5 and wherein saidfuel channel has a square cross section;and wherein said rods aredisposed in rows and columns and wherein said partial length rod is onerow removed from the wall of said channel.
 8. The invention of claim 7and wherein said rod array includes 8×8 total rods and wherein aplurality of partial length rods are located at least one row removedfrom said channel wall.
 9. The invention of claim 5 and wherein saidrods include 9 rows and 9 columns of rods and said partial length rodsare located at least one row removed from said channel wall.
 10. Animproved fuel bundle assembly for a boiling water reactor comprising incombination:a fuel channel having vertically extending walls forming acontinuous closed channel around said fuel assembly, said channel beingopen at the bottom end for engagement to a lower tie plate and open atthe upper end for engagement to an upper tie plate; a plurality of rowsand columns of fuel rods for placement within said fuel channel, eachsaid rod containing fissile material for producing nuclear reactionswhen in the presence of sufficient moderated neutron flux; a lower tieplate for supporting said rods within said channel, said lower tie plateat the bottom of said channel, said lower tie plate further providingdefined apertures for the inflow of water to said channel between saidrods, said water being heated by said rods during said nuclear reactionto include heating of the water in a single phase region adjacent saidlower tie plate and heating of the water to generate steam from saidwater in a two phase region overlying said lower tie plate and adjoiningan upper tie plate; an upper tie plate for supporting the upper end ofsaid plurality of rows and columns of said fuel rods, said upper tieplate further providing defined apertures for the outflow of water andsteam from said channel between said rods; spacers intermediate saidupper and lower tie plates at preselected elevations along said fuelrods for maintaining said rows and columns of fuel rods in spaced apartrelation along the length of said fuel assembly; at least one of saidspacers being located in said two phase region; a plurality of said fuelrods being partial length fuel rods extending from said lower tie platetoward said upper tie plate, said partial length rods terminating abovea spacer within the two phase region of said bundle before reaching saidupper tie plate; at least two of said partial length rods being separatefrom one another so as to define in at least two locations in saidbundle spaced apart and separate vents commencing at the top of saidpartial length rods and extending to said upper tie plate, each of saidspaced apart vents being immediately adjoined by adjacent full lengthfuel rods;
 11. The invention of claim 10 and wherein said upper tieplate overlying said partial length rod includes an enlarged aperturefor flow of water and steam.
 12. The invention of claim 10 wherein fuelrod spacers above and overlying said partial length rod include enlargedapertures in the flow path above said part length rod for flow of waterand steam.
 13. In a boiling water reactor having discrete bundles offuel rods confined within channel enclosed fuel assemblies, an improvedfuel bundle comprising:a fuel channel having vertically extending wallsforming a continuous channel around a fuel assembly volume, said channelbeing open at the bottom end for engagement to a lower tie plate andopen at the upper end for engagement to an upper tie plate; a pluralityof fuel rods for placement within said channel, each said fuel rodcontaining fissile material for producing nuclear reaction when in thepresence of sufficient moderated neutron flux; a lower tie plate forsupporting said bundle of rods within said channel, said lower tie platejoining the bottom of said channel to close said channel, said lower tieplate providing defined apertures for the inflow of water in saidchannel between said rods for the generation of steam during saidnuclear reaction; said plurality of fuel rods extending from said lowertie plate wherein a single phase region of said water in said bundle isdefined to an upward portion of said bundle wherein a two phase regionof said water and steam in said bundle is defined during nuclear steamgenerating reaction in said fuel bundle; an upper tie plate forsupporting the upper end of said bundle of rods, said upper tie platejoining the top of said channel to close the top end of said channel,said upper tie plate providing defined apertures for the outflow ofwater and steam in said channel from the generation of steam duringnuclear reaction; at least one large rod extending in said fuel bundlehaving moderator contained therein from providing to said upper twophase region of said bundle additional moderator for moderating reactionproduced fast neutrons to reaction continuing thermal neutrons, saidlarge water rod occupying a portion of the rod positions in said rodmatrix; a plurality of said fuel rods being part length fuel rodsextending from said lower tie plate towards said upper tie plate, saidpartial length rods terminating within the two phase region of saidbundle before reaching said upper tie plate; at least two of saidpartial length rods being separate from one another so as to define inat least two locations in said bundle spaced apart and separate ventscommencing at the top of said partial length rods and extending to saidupper tie plate; each of said spaced apart vents being immediateadjoined by adjacent full length fuel rods and being independent of saidat least one large rod providing moderator to the upper two phase regionof said bundle; whereby during steam generation in said fuel bundlesteam vents along a plurality of separate paths overlying said partiallength fuel rods.
 14. The invention of claim 13 and wherein said partlength rods are all the same length.
 15. The invention of claim 13 andwherein said large rod containing moderator is a large water rod. 16.The invention of claim 13 and wherein said part length rods have onefull length rod between said part length rod and said channel.
 17. In aboiling water reactor having discrete bundles of fuel rods confinedwithin channel enclosed fuel assemblies, the channel enclosed fuelassemblies being of the type including;a fuel channel having verticallyextending walls forming a continuous channel around a fuel assemblyvolume, said channel being open at the bottom end for engagement to alower tie plate and open at the upper end for engagement to an upper tieplate; a plurality of fuel rods for placement within said channel, eachsaid fuel rod containing fissile material for producing nuclear reactionwhen in the presence of sufficient moderated neutron flux; a lower tieplate for supporting said bundle of rods within said channel, said lowertie plate joining the bottom of said channel to close said channel, saidlower tie plate providing defined apertures for the inflow of water insaid channel between said rods for the generation of steam during saidnuclear reaction; said plurality of fuel rods extending from said lowertie plate wherein a single phase region of said water in said bundle isdefined to an upward portion of said bundle wherein a two phase regionof said water and steam in said bundle is defined during nuclear steamgenerating reaction in said fuel bundle; an upper tie plate forsupporting the upper end of said bundle of rods, said upper tie platejoining the top of said channel to close the top end of said channel,said upper tie plate providing defined apertures for the outflow ofwater and steam in said channel from the generation of steam duringnuclear reaction; at least one large rod extending in said fuel bundlehaving moderator contained therein from providing to said upper twophase region of said bundle additional moderator for moderating reactionproduced fast neutrons to reaction continuing thermal neutrons, saidlarge rod providing moderator occupying a portion of the rod positionsin said rod matrix; the improvement to said fuel bundle comprising: aplurality of said fuel rods being part length fuel rods extending fromsaid lower tie plate towards said upper tie plate, said partial lengthrods terminating within the two phase region of said bundle beforereaching said upper tie plate; at least two of said partial length rodsbeing separate from one another so as to define in at least twolocations in said bundle spaced apart and separate vents commencing atthe top of said partial length rods and extending to said upper tieplate; each of said spaced apart vents being immediate adjoined byadjacent full length fuel rods and being independent of said at leastone large rod providing moderator to the upper two phase region of saidbundle; whereby during steam generation in said fuel bundle steam ventsalong a plurality of separate paths overlying said partial length fuelrods.
 18. The invention of claim 17 and wherein said at least one largerod providing moderator in the upper two phase region of said fuelbundle is a large water rod.
 19. The invention of claim 17 and whereinsaid part length fuel rods are all the same length.
 20. The invention ofclaim 17 and wherein said part length fuel rods have reduced fissilematerial at the ends of said rods.
 21. The invention of claim 17 andwherein said at least one large rod providing moderator in the upper twophase region of said fuel bundle contains water moderator.
 22. In aboiling water reactor having discrete bundles of fuel rods confinedwithin channel enclosed fuel assemblies, an improved fuel bundlecomprising:a fuel channel having vertically extending walls forming acontinuous channel around a fuel assembly volume, said channel beingopen at the bottom end for engagement to a lower tie plate and open atthe upper end for engagement to an upper tie plate; a plurality of fuelrods for placement within said channel, each said fuel rod containingfissile material for producing nuclear reaction when in the presence ofsufficient moderated neutron flux; a lower tie plate for supporting saidbundle of rods within said channel, said lower tie plate joining thebottom of said channel to close said channel, said lower tie plateproviding defined apertures for the inflow of water in said channelbetween said rods for the generation of steam during said nuclearreaction; said plurality of fuel rods extending from said lower tieplate wherein a single phase region of said water in said bundle isdefined to an upward portion of said bundle wherein a two phase regionof said water and steam in said bundle is defined during nuclear steamgenerating reaction in said fuel bundle; an upper tie plate forsupporting the upper end of said bundle of rods, said upper tie platejoining the top of said channel to close the top end of said channel,said upper tie plate providing defined apertures for the outflow ofwater and steam in said channel from the generation of steam duringnuclear reaction; a plurality of spacers placed along the length of saidbundle, said spacers surrounding said fuel rods for maintaining saidfuel rods in uniform side by side relation along the length of said fuelbundle; a plurality of said fuel rods being part length fuel rodsextending from said lower tie plate towards said upper tie plate, saidpartial length rods terminating within the two phase region of saidbundle before reaching said upper tie plate; at least two of saidpartial length rods being separate from one another so as to define inat least two locations in said bundle spaced apart and separate ventscommencing at the top of said partial length rods and extending to saidupper tie plate; each of said spaced apart vents being immediateadjoined by adjacent full length fuel rods providing moderator to theupper two phase region of said bundle; said spacers above said partlength fuel rods defining apertures overlying part length fuel rods forpermitting unobstructed upward flow of said vents; whereby during steamgeneration in said fuel bundle steam vents along a plurality of separatepaths overlying said partial length fuel rods.
 23. The invention ofclaim 22 and wherein said one of said spacers is immediate adjacent anend of said part length rod.
 24. The invention of claim 22 and whereinsaid upper tie plate overlying said part length rods includes aperturesfor permitting upward unobstructed venting of said vents.
 25. In aboiling water reactor having discrete bundles of fuel rods confinedwithin channel enclosed fuel assemblies, an improved fuel bundlecomprising:a fuel channel having vertically extending walls forming acontinuous channel around a fuel assembly volume, said channel beingopen at the bottom end for engagement to a lower tie plate and open atthe upper end for engagement to an upper tie plate; a plurality of fuelrods for placement within said channel, each said fuel rod containingfissile material for producing nuclear reaction when in the presence ofsufficient moderated neutron flux; a lower tie plate for supporting saidbundle of rods within said channel, said lower tie plate joining thebottom of said channel to close said channel, said lower tie plateproviding defined apertures for the inflow of water in said channelbetween said rods for the generation of steam during said nuclearreaction; said plurality of fuel rods extending from said lower tieplate wherein a single phase region of said water in said bundle isdefined to an upward portion of said bundle wherein a two phase regionof said water and steam in said bundle is defined during nuclear steamgenerating reaction in said fuel bundle; an upper tie plate forsupporting the upper end of said bundle of rods, said upper tie platejoining the top of said channel to close the top end of said channel,said upper tie plate providing defined apertures for the outflow ofwater and steam in said channel from the generation of steam duringnuclear reaction; a plurality of said fuel rods being part length fuelrods extending from said lower tie plate towards said upper tie plate,said partial length rods terminating within the two phase region of saidbundle before reaching said upper tie plate; at least two of saidpartial length rods being separate from one another so as to define inat least two locations in said bundle spaced apart and separate ventscommencing at the top of said partial length rods and extending to saidupper tie plate; each of said spaced apart vents being immediateadjoined by adjacent full length fuel rods providing moderator to theupper two phase region of said bundle; said tie plate above said partlength fuel rods defining apertures overlying part length fuel rods forpermitting unobstructed upward flow of said vents; whereby during steamgeneration in said fuel bundle steam vents along a plurality of separatepaths overlying said partial length fuel rods.
 26. The invention ofclaim 25 and further including:a plurality of spacers disposed atvertical intervals between said fuel rods and extending to said channelfor holding said fuel rods in parallel side by side relation, saidspacers overlying the ends of said partial length rods having aperturesfor permitting upward unobstructed flow of said vents.
 27. An upper tieplate for a boiling water reactor having discrete bundles of fuel rodsconfined within channel enclosed fuel assemblies, said channel enclosedfuel assemblies of the class having:a fuel channel having verticallyextending walls forming a continuous channel around a fuel assemblyvolume, said channel being open at the bottom end for engagement to alower tie plate and open at the upper end for engagement to an upper tieplate; a plurality of fuel rods for placement within said channel, eachsaid fuel rod containing fissile material for producing nuclear reactionwhen in the presence of sufficient moderated neutron flux; a lower tieplate for supporting said bundle of rods within said channel, said lowertie plate joining the bottom of said channel to close said channel, saidlower tie plate providing defined apertures for the inflow of water insaid channel between said rods for the generation of steam during saidnuclear reaction; said plurality of fuel rods extending from said lowertie plate wherein a single phase region of said water in said bundle isdefined to an upward portion of said bundle wherein a two phase regionof said water and steam in said bundle is defined during nuclear steamgenerating reaction in said fuel bundle; an upper tie plate forsupporting the upper end of said bundle of rods, said upper tie platejoining the top of said channel to close the top end of said channel,said upper tie plate providing defined apertures for the outflow ofwater and steam in said channel from the generation of steam duringnuclear reaction; a plurality of said fuel rods being part length fuelrods extending from said lower tie plate towards said upper tie plate,said partial length rods terminating within the two phase region of saidbundle before reaching said upper tie plate; at least two of saidpartial length rods being separate from one another so as to define inat least two locations in said bundle spaced apart and separate ventscommencing at the top of said partial length rods and extending to saidupper tie plate; each of said spaced apart vents being immediateadjoined by adjacent full length fuel rods providing moderator to theupper two phase region of said bundle; the improvement to said upper tieplate comprising: said upper tie plate above said part length fuel rodsdefining apertures overlying part length fuel rods for permittingunobstructed upward flow of said vents; whereby during steam generationin said fuel bundle steam vents along a plurality of separate pathsoverlying said partial length fuel rods.
 28. The invention of claim 27and wherein said discrete bundles of fuel rods are of the type includingspacers spaced vertically along the length of said fuel bundles aroundsaid fuel rods within said channels and the improvement to said fuelbundle further comprises:said spacers over the ends of said partiallength rods defining enlarged apertures for permitting upwardunobstructed passage of said vents, said apertures in said spacersaligned with said apertures in said tie plate.
 29. A spacer for aboiling water reactor having discrete bundles of fuel rods confinedwithin channel enclosed fuel assemblies, said channel enclosed fuelassemblies of the class having:a fuel channel having verticallyextending walls forming a continuous channel around a fuel assemblyvolume, said channel being open at the bottom end for engagement to alower tie plate and open at the upper end for engagement to an upper tieplate; a plurality of fuel rods for placement within said channel, eachsaid fuel rod containing fissile material for producing nuclear reactionwhen in the presence of sufficient moderated neutron flux; a lower tieplate for supporting said bundle of rods within said channel, said lowertie plate joining the bottom of said channel to close said channel, saidlower tie plate providing defined apertures for the inflow of water insaid channel between said rods for the generation of steam during saidnuclear reaction; said plurality of fuel rods extending from said lowertie plate wherein a single phase region of said water in said bundle isdefined to an upward portion of said bundle wherein a two phase regionof said water and steam in said bundle is defined during nuclear steamgenerating reaction in said fuel bundle; an upper tie plate forsupporting the upper end of said bundle of rods, said upper tie platejoining the top of said channel to close the top end of said channel,said upper tie plate providing defined apertures for the outflow ofwater and steam in said channel from the generation of steam duringnuclear reaction; a plurality of spacers around said fuel rods andwithin said channels for maintaining said fuel rods in vertical side byside relation; a plurality of said fuel rods being part length fuel rodsextending from said lower tie plate towards said upper tie plate, saidpartial length rods terminating within the two phase region of saidbundle before reaching said upper tie plate; at least two of saidpartial length rods being separate from one another so as to define inat least two locations in said bundle spaced apart and separate ventscommencing at the top of said partial length rods and extending to saidupper tie plate; each of said spaced apart vents being immediateadjoined by adjacent full length fuel rods providing moderator to theupper two phase region of said bundle; the improvement to said spacerscomprising: said spacers above said part length fuel rods definingapertures overlying part length fuel rods for permitting unobstructedupward flow of said vents; whereby during steam generation in said fuelbundle steam vents along a plurality of separate paths overlying saidpartial length fuel rods.
 30. The invention of claim 29 and includingthe improvement of:said tie plate above said part length fuel rodsdefining apertures overlying part length fuel rods for permittingunobstructed upward flow of said vents; and, said defined apertures ofsaid tie plate registered to said defined apertures in said spacers.